It is known to produce transistors within a layer of semiconductor material that is electrically insulated from the substrate by an intermediate oxide layer. A circuit that incorporates such transistors consumes less energy thanks to the elimination of leakage currents between the transistors and the substrate, the latter generally being connected to an electrical earth of the circuit. Such a circuit is known by the name SOI (Silicon On Insulator).
In an SOI circuit, control of the conduction of a transistor by the gate is degraded when the level of integration of the circuit increases. This degradation stems mainly from short channel effects or SCE, which are due to the thickness of the layer of semiconductor material within which the transistor is produced. When the length of the channel, measured in the direction of conduction of the transistor, is less than 0.05 microns, the short channel effects are perceptible for a thickness of the layer of semiconductor material of greater that 15 nanometers.
Furthermore, in substrates currently available for producing SOI circuits, the thickness of the layer of semiconductor material is not controlled to better than 10 nanometers. This results in dispersion of the electrical characteristics of the transistors fabricated at various places on a specified substrate, especially a dispersion of the threshold voltages.
Moreover, the currently available substrates for producing SOI-type transistors possess a relatively thick insulating intermediate layer. The thickness of this layer varies between 145 and 400 nanometers. The implantation into the substrate of electrically doping species, so as to give it a suitable electrical conductivity, is therefore difficult through the two layers of semiconductor material and of insulating material. The effect of the earth plane created with respect to the transistors by this implantation beneath the insulating intermediate layer is consequently reduced, especially because of the increased distance between each transistor and the substrate. The short channel effects are therefore increased.